Engine control system and method with lean catalyst and particulate filter

ABSTRACT

A method for controlling reductant added to an exhaust of an engine having a lean NOx catalyst and a particulate filter adjusts a reductant amount during particulate filter regeneration. The method adjusts the reductant amount to account for reducing agents released from the particulate filter that are experienced by the lean NOx catalyst. In addition, management of particulate filter regeneration is based on both an estimated amount of stored particles and conditions of the lean NOx catalyst. In this way, operation of both the particulate filter and the lean NOx catalyst can be optimized. Also, termination of particulate filter regeneration is determined based on operating conditions.

FIELD OF THE INVENTION

The present invention relates to a system and method for controlling anengine having both a lean NOx catalyst for reducing exhaust NOx in anoxygen rich environment and a particulate filter for removing carbonparticles, or soot.

BACKGROUND OF THE INVENTION

In order to meet future emission regulations in vehicles having acompression ignition engine, it may be necessary to use lean NOxcatalysts in combination with particulate filters.

A NOx catalyst reduces NOx emissions continuously, even in an oxygenrich environment. For an active NOx catalyst to maximize NOx reduction,a reducing agent, for example, diesel fuel or urea, needs to be present.The optimum amount of reducing agent for the NOx catalyst is typicallybased on engine operating conditions and catalyst conditions. Theseconditions typically include engine speed, engine load, and catalysttemperature.

A particulate filter, also commonly used with compression ignitionengines, is used to prevent soot, or carbon particles, from exiting thetailpipe. Since the particulate filter has a limited storage capacity,it is periodically regenerated. In one approach, during the regenerationprocess, exhaust temperature is increased to ignite carbon particlesstored in the particulate filter. By burning the stored carbonparticles, the filter is regenerated and able to again store the carbonparticles. In addition, the burning of the carbon particles causes anincrease in temperature.

One approach for managing a particulate filter estimates that amount ofstored particulates and then regenerates the filter when this amountreaches a predetermined value. This decision can be augmented based on avehicle operating zone defined by engine speed and load. Such a systemis describe by EP 0 859 132.

The inventor herein has recognized numerous disadvantages when usingprior particulate filter management systems when a NOx catalyst ispresent. In particular, the above management system does not considerthe state of the NOx catalyst when determining filter regeneration. Forexample, the prior art does not consider conditions where heat generatedfrom regeneration raise NOx catalyst temperature beyond appropriatelimits.

SUMMARY OF THE INVENTION

An object of the present invention is to manage particulate regenerationsystem where an exhaust stream of a compression ignition engine iscoupled to both a particulate filter and a lean NOx catalyst.

The above object is achieved and disadvantages of prior approachesovercome by a method for managing regeneration of a particulate filtercommunicating with an engine exhaust upstream of a catalyst, the methodcomprising: estimating an amount of stored particulates in theparticulate filter; and regenerating the particulate filter dependentupon said amount and a condition of the catalyst.

By including catalyst conditions in regeneration of the particulatefilter, it is possible coordinate operation of both the particulatefilter and the catalyst, thereby improving performance of the overallsystem. Further, it is possible to avoid degradation of the catalystusing a catalyst condition in controlling regeneration of theparticulate filter.

An advantage of the above aspect of the present invention is improvedsystem durability.

Another advantage of the above aspect of the present invention isimproved system performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages described herein will be more fullyunderstood by reading an example of an embodiment in which the inventionis used to advantage, referred to herein as the Description of PreferredEmbodiment, with reference to the drawings, wherein:

FIGS. 1A and 1B schematic diagrams of an engine wherein the invention isused to advantage; and

FIGS. 2-7 are high level flow charts of various operations performed bya portion of the embodiment shown in FIGS. 1A and 1B.

DESCRIPTION OF PREFERRED EMBODIMENT(S)

Internal combustion engine 10, comprising a plurality of cylinders, onecylinder of which is shown in FIG. 1A, is controlled by electronicengine controller 12. Engine 10 includes combustion chamber 30 andcylinder walls 32 with piston 36 positioned therein and connected tocrankshaft 40. Combustion chamber 30 is known communicating with intakemanifold 44 and exhaust manifold 48 via respective intake valve 52 andexhaust valve 54. Intake manifold 44 is also shown having fuel injector80 coupled thereto for delivering liquid fuel in proportion to the pulsewidth of signal FPW from controller 12. Both fuel quantity, controlledby signal FPW and injection timing are adjustable. Fuel is delivered tofuel injector 80 by a diesel fuel system (not shown) including a fueltank, fuel pump, and fuel rail (not shown). Alternatively, the enginemay be configured such that the fuel is injected directly into thecylinder of the engine, which is known to those skilled in the art as adirect injection engine.

Reducing agent, for example, ammonia or diesel fuel, is stored instorage vessel 130 coupled to exhaust manifold 48 upstream ofparticulate filter 95 and lean NOx catalyst 97. In an alternativeembodiment (not show) diesel fuel can be stored solely in the fuel tankand supplied to the exhaust system. Also, catalyst 97 is lean NOxcatalyst capable of reducing NOx in an oxygen rich environment.Efficiency of catalyst 97 is increased in the presence of a reducingagent.

Control valve 134 controls the quantity of reducing agent delivered tothe exhaust gases entering catalyst 97. Pump 132 pressurizes thereducing agent supplied to control valve 134. Both Pump 132 and controlvalve 134 are controlled by controller 12. Ammonia sensor 140 is showncoupled to exhaust manifold 48 downstream of catalyst 97. Temperaturesensor 142 coupled to catalyst 97 provides an indication of thetemperature (T) of catalyst 97. Alternatively, catalyst temperature (T)can be estimated as described later herein with particular reference toFIG. 6. Similarly, particulate filter temperature (Tp) can be read fromsensor 143 or estimated using methods known to those skilled in the artbased on exhaust gas temperature.

Particulate filter 95 is capable of storing carbon particles from theexhaust. Particulate filter 95 can be regenerated by increasingtemperature (Tp) to a point where the stored particles ignite and burnaway. Particulate filter 95 is a standard particulate filter as is knownto those skilled in the art.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-onlymemory 106, random access memory 108, and a conventional data bus.Controller 12 is shown receiving various signals from sensors coupled toengine 10, in addition to those signals previously discussed, including:engine coolant temperature (ECT) from temperature sensor 112 coupled tocooling sleeve 114; a measurement of manifold pressure (MAP) frompressure sensor 116 coupled to intake manifold 44; a measurement (AT) ofmanifold temperature from temperature sensor 117; an engine speed signal(RPM) from engine speed sensor 118 coupled to crankshaft 40.

Referring now to FIG. 1B, an alternative embodiment shown where engine170 is a direct injection engine with injector 80 located to inject fueldirectly into cylinder 30. In this example, reductant, or diesel fuel,is delivered to the exhaust system by injector 80 during either or bothof a power or exhaust stroke of engine 170.

Referring now to FIG. 2, a routine for controlling injection of areductant into exhaust flow is described. First, in step 210, theinitial reductant injection amount (ra_i) is calculated based on engineoperating conditions. These conditions include conditions known to thoseskilled in the art to indicate the amount of nitrogen oxide produced bythe combustion process. These conditions include: engine speed, engineload, exhaust temperatures, and catalyst temperatures. Other conditions,such as injection timing, engine temperature, and any other parameterknown to those skilled in the art to affect engine nitrogen oxideproduction, can also be used. Next, in step 212, a determination is madeif the particulate filter is currently in the regeneration process, asdescribed later herein with particular reference to FIG. 3. When theanswer to step 212 is YES, the routine continues to step 216, whereinitial reductant amount (ra_i) is adjusted by adjustment amount Ara.Adjustment amount (Δra) is determined as described later herein withparticular reference to FIG. 6. In particular, adjustment amount (Δra)is determined based on the amount of reductant released from theparticulate filter during regeneration of the particulate filter.Otherwise, in step 214, adjustment of the initial reductant amount isnot required.

Those skilled in the art will recognize that there are many alternateembodiments to the previous steps of determining an initial reductantamount and an adjustment amount. For example, one equivalent alternativeis to use two separate calculations for determining a reductantinjection amount, one during regeneration and the other duringnon-regenerating conditions. This, or any other method that addsreductant to the exhaust dependent on particulate filter regeneration,can therefore be equivalently used in the present invention.

Continuing with FIG. 2, the injection of reductant, through either valve134 or through late injection through injector 80 in FIG. 1B, iscontrolled at step 218 based on adjusted reductant amount (ra_a) asdetermined in either step 214 or step 216. In this way, an optimumamount of reductant can be injected, including compensation forparticulate filter regeneration. The reductant can be injected in step218 in many ways, including: blade injection by a mean fuel injector inthe combustion chamber so that fuel is injected during the exhauststroke, external reductant injection where the reductant is injecteddirectly into the exhaust stream, or any other method known to thoseskilled in the art for providing reductant to a catalyst.

Referring now to FIG. 3, the routine for determining if regeneration ofthe particulate filter is required is described. First, in step 310, theamount of stored particulate (spa) is determined, as described laterherein with particular reference to FIG. 5. Next, in step 310, thedetermination is made as to whether regeneration is required bycomparing stored particulate amount (spa) to a first particulatethreshold (S1). When the answer to step 312 is YES, the routinecontinues to step 314 where engine parameters are controlled to increaseexhaust temperature, thereby allowing regeneration of the particulatefilter.

Any method known to those skilled in the art for increasing exhaust gastemperature of a compression ignition engine can be used such as, forexample, throttling the engine intake, increasing exhaust gasrecirculation amount, adjusting injection timing, or combusting fuelduring an exhaust stroke of the engine. Next, in step 316, theregeneration flag is set.

Referring now to FIG. 4, an alternate routine for determining whetherregeneration of the particulate filter is required is described. First,in step 410, the amount of stored particulates (spa) is determined, asdescribed later herein with particular reference to FIG. 5. Then, instep 412, the particulate filter temperature (tp) is estimated. In apreferred embodiment, the particulate filter temperature is estimatedbased on engine operating conditions using characteristic predeterminedmaps stored in memory. The engine operating parameters used compriseengine speed, fuel injection amount, fuel injection timing, and enginetemperature. Any other method known to those skilled in the art forestimating temperature of an emission control device can be used toadvantage with the present invention. Next, in step 412, the determinedis made as to whether particulate filter regeneration is required. Inparticular, it is determined in step 414 whether stored particulateamount (spa) is greater than limit amount S2 and particulate filtertemperature is greater than temperature limit T2, or whether storedparticulate amount (spa) is greater than limit amount Sl, or whethercatalyst temperature (Tc) is less than temperature limit T3 and storedparticulate amount (spa) is greater than limit amount S3.

In one aspect of the present invention, temperature limit T3 representsa light off temperature. Thus, according to the present invention, whencatalyst 97 is below a light off temperature and there are enough storedparticulates to burn (limit S3), regeneration is used to increasecatalyst temperature Tc.

In another aspect of the present invention, when temperature limit T2below regeneration temperature and limit amount S2 represents an amountof stored particles less than S1, but greater than S3. Thus, the presentinvention takes advantage of high filter temperature that may beencountered during certain driving conditions by purging storedparticulates at this time. Thus, only a small amount of energy is usedto increase the filter temperature to the regeneration temperature,thereby increasing fuel economy by opportunistically regenerating thefilter.

Also in this example, limit S1 is greater than S2. In this way, theroutine takes advantage of situations where little fuel economy is lostto regenerate the particulate filter. For example, even when theparticulate filter is not completely full but the particulate filtertemperature is very close to the regeneration temperature, only a smallamount of energy is needed to bring the particulate filter to theregeneration temperature thereby efficiently regenerating theparticulate filter. When the answer to step 414 is YES, the routinecontinues to step 416 where exhaust gas temperature is increased. Next,in step 418, the regeneration flag is set.

Referring now to FIG. 5, a routine is described for determining storedparticulates. First, in step 510, a determination is made as to whetherthe particulate filter is currently in the regeneration process, forexample, by checking the regeneration flags in step 418. When the answerto step 510 is YES, the routine continues to step 512. In step 512, theroutine determines the stored particulate amounts during regeneration byincluding the particulates generated by the combustion process (cpa),the current stored particulate amount (spa), and the amount ofparticulates released during the regeneration stage (rpa). Otherwise,the routine moves to step 514 and determines the stored particulateamount based on the current stored particulate amount and theparticulates produced during the combustion process. In a preferredembodiment, the amount of particulates generated during the combustionprocess (cpa) is determined based on engine operating conditions such asfuel injection amount and engine speed. Also, the amount of releasedparticulates during the regeneration process (rpa) is determined basedon exhaust gas space velocity and particulate filter temperature (tp).

Referring now to FIG. 6, a routine for calculating the reductant amountadjustment (Ara) is described. First, in step 610, the catalysttemperature (Tc) is estimated based on engine operating conditions. Inparticular, catalyst temperature (Tc) is estimated based on a normalestimated temperature (Tn) based on engine operating conditions thatrepresents catalyst temperature under normal conditions. Catalysttemperature (Tc) is also estimated based on a delta temperature (DT)that represents the additional temperature due to the heat generated bythe particular filter regeneration. In an alternative embodiment,catalyst temperature (Tc) can be read from sensor 142 if available.Also, particulate filter temperature (Tp) can be estimated based onengine operating conditions. Further, (Tp) represents the actual, orestimated temperature, while (Tpn) represents an estimated particulatefilter temperature that would be obtained without the purposely changingengine operating conditions to increase heat to the exhaust system.

Next, in step 612, the reductant amount adjustment (Ara) is determinedbased on a function (f) of delta temperature (DT) and the amount ofparticulates released during the regeneration process (rpa). Function fis highly dependent on catalyst formulation. In particular, the amountof available reducing agent produced during the regeneration processesdepends on whether the catalyst formulation can make use of the productsof regeneration. For example, during the regeneration process, CO may beproduced. If the catalyst formulation is such that CO can be used toreduce NOx, then the amount of reducing agent to be injected (ra) mustbe adjusted based on the amount of CO released during regeneration.However, if the catalyst material cannot make good use of CO, then adifferent amount of adjustment is made to signal (ra). In an alternativeembodiment, another function, h, can be used where catalyst temperature(Tc) is used rather than delta temperature (DT).

Continuing with FIG. 6, in step 614, a determination is made as towhether adjusted reductant amount (ra_a) as determined in step 216 isless than zero. When the answer is YES, regeneration is discontinued andregeneration flag is un-set in step 616. In an alternate embodiment (notshow), a determination as to whether adjusted reductant amount (ra_a) asdetermined in step 216 is less than a predetermined value (PV1) can beused, where the predetermined value is used to give more flexibilityrather than strictly using zero. Thus, a determination is made as towhether it is possible to maintain the correct total amount of reductantneeded to maximize efficiency of catalyst 97. When so much reductant isbeing released by filter 95 during regeneration that even by completelydiscontinuing reductant addition, there is still excess reductant,regeneration is discontinued. For example, the predetermined value (PV1)can represent a maximum amount of excess reductant tolerated in catalyst97 (thereby being a negative value used in the comparison of step 614).

Those skilled in the art will recognize that various other alternativeembodiments can be used to yield a similar result. For example, whenexternal reductant is not added, particulate filter regeneration can bediscontinued when an amount of reductant released during theregeneration process reaches a predetermined maximum value. In otherwords, when an amount of reductant released during regeneration isgreater than that which can be utilized by catalyst 97, regeneration isterminated. Regeneration can be terminated in many way, for example, bydiscontinuing elevation of exhaust temperature, or by intentionallycooling exhaust gas using injection timing, or any other controlvariable known to those skilled in the art.

In another alternative embodiment, reductant amount adjustment (Δra) canbe compared to base reductant amount (ra) and when (Δra) is greater than(ra), regeneration terminated. Those skilled in the art will recognizethis as an alternative embodiment of step 614 as described previouslyherein.

Referring now to FIG. 7, a routine for deactivating particulate filterregeneration is described. First, in step 710, a determination is madeas to whether the particulate filter is currently in the regenerationprocess, for example, by checking the regeneration flags in step 418.When the answer to step 710 is YES, a determination is made in step 712as to whether stored particulate amount (spa) is less than limit amountS4, or whether catalyst temperature (Tc) is greater than limit T5, orwhether particulate temperature during non regeneration operation (Tpn)is less than limit T6 and stored particulate amount (spa) is less thanlimit S6.

In one aspect of the present invention, limit amount S4 represents whenparticulate filter 95 is regenerated. Thus, the regeneration cancontinue until filter 95 is fully regenerated. In another aspect of thepresent invention, temperature limit T5 represents a maximum temperaturelimit above which catalyst degradation can occur. Thus, regeneration isdiscontinued to reduce exhaust temperatures so that degradation ofcatalyst 97 is avoided. In yet another aspect of the present invention,temperature limit T6 represents a temperature at which filter 95 wouldnormally operate without regeneration. Thus, if the amount of heat addedto sustain regeneration is too large and the stored particulate amount(spa) indicates through comparison with limit S6 that there is a certainamount of storage capability, regeneration is terminated. Thus, improvedfuel economy can be achieved while minimizing emissions.

Continuing with FIG. 7, when the answer to step 712 is YES, the routinecontinues to step 714 where regeneration is deactivated and regenerationflag unset.

This concludes the description of the Preferred Embodiment. The readingof it by those skilled in the art would bring to mind many alterationsand modifications without departing from the spirit and scope of theinvention. Accordingly, it is intended that the scope of the inventionbe limited only by the following claims.

What is claimed is:
 1. A method for managing regeneration of a particulate filter communicating with an engine exhaust upstream of a catalyst, the method comprising: estimating an amount of stored particulates in the particulate filter; regenerating the particulate filter dependent upon said amount and a condition of the catalyst, adding a first reductant amount to the exhaust when regenerating the particulate filter; and adding a second reductant amount to the exhaust otherwise, wherein said first reductant amount is based on a quantity of released reductant from regeneration of the particulate filter.
 2. The method recited in claim 1 wherein said condition is a catalyst temperature.
 3. The method recited in claim 1 further comprising the steps of: regenerating the particulate filter by increasing an exhaust temperature when said amount of stored particulates is greater than a predetermined amount; and discontinuing said regeneration when said catalyst temperature is greater than a preselected value.
 4. The method recited in claim 3 wherein said preselected value is a maximum allowable temperature without catalyst degradation.
 5. The method recited in claim 1 further comprising the step of regenerating the particulate filter by increasing an exhaust temperature when said amount of stored particulates is greater than a first minimum amount and said catalyst temperature is less than a second minimum value.
 6. The method recited in claim 5 wherein said second minimum value is a light off temperature above which maximum catalyst efficiency is achievable.
 7. The method recited in claim 1 further comprising the steps of: regenerating the particulate filter by increasing an exhaust temperature when said amount of stored particulates is greater than a predetermined amount; and discontinuing said regeneration when an exhaust temperature that would be experienced during non-regeneration conditions is below a first preselected value and said stored particulate amount is below a second preselected value.
 8. The method recited in claim 7 wherein said exhaust temperature represents a particulate filter temperature that would be experienced during non-regeneration conditions.
 9. The method recited in claim 1 further comprising the step of regenerating the particulate filter based on said amount and a temperature of the particulate filter.
 10. The method recited in claim 1 further comprising the step of regenerating the particulate filter by increasing an exhaust temperature when said amount of stored particulates is greater than a first minimum amount and said catalyst temperature is greater than a first threshold value.
 11. The method recited in claim 1 wherein said catalyst is a lean NOx catalyst capable of reducing NOx in an oxygen rich environment.
 12. The method recited in claim 1 wherein said first reductant amount is based on an increased catalyst temperature from heat generated by regeneration of the particulate filter.
 13. A method for managing regeneration of a particulate filter communicating with an engine exhaust upstream of an emission control device, the method comprising: estimating an amount of stored particulates in the particulate filter; regenerating the particulate filter when said amount is greater than a first predetermined amount and based on an emission control device operating condition, wherein said regeneration further comprises: regenerating the particulate filter when said amount of stored particulates is greater than a first predetermined amount; regenerating the particulate filter when said amount of stored particulates is greater than a second predetermined amount and a catalyst temperature is greater than a third predetermined amount; and regenerating the particulate filter when said amount of stored particulates is greater than a fourth predetermined amount and a catalyst temperature is greater than a fifth predetermined amount.
 14. The method recited in claim 13 wherein said emission control device operating condition is a lean NOx catalyst temperature.
 15. The method recited in claim 13 wherein said emission control device operating condition is an exhaust temperature.
 16. The method recited in claim 13 wherein said emission control device operating condition is a particulate filter temperature.
 17. The method recited in claim 15 further comprising the steps of: discontinuing said regeneration when said catalyst temperature is greater than a first preselected value; discontinuing said regeneration when said amount of stored particulates is less than a second preselected amount; and discontinuing said regeneration when an exhaust temperature that would be experienced during non-regeneration conditions is below a third preselected value and said stored particulate amount is below a fourth preselected value. 